1. Field of the Invention
The present invention relates to a biodegradable thermoplastic polyester resin and a process for preparing and/or producing the same. More particularly, the present invention relates to a high molecular weight biodegradable polyester resin which can be used in medical, agricultural packaging materials by enhancing the low heat-resistance of the general aliphatic polyester.
2. Description of the Background Art
The typical polyester resin used for various purposes such as textures and fibers, moldings and forming, or films, is a high molecular weight aromatic polyester resin produced by the polycondensation reaction of terephthalic acid with ethyleneglycol, or terephthalic acid with 1,4-butanediol. The high molecular weight polyester resin is a polymer having a number average molecular weight of over 10,000. A polyester resins are not degradable naturally so it is a serious, worldwide of environmental concern.
Otherwise, the aliphatic polyester resins are known as being biodegradable (Journal of Marcomol, SCI-Chem., A-23(3), 1986, 393-409). They have a variety of usage and in the medical and agricultural fields, and other applications are being developed.
However, the conventional aliphatic polyester resin has a low melt point and a high melting index, because of the structure of the main chain and crystallinity, and having low heat resistance and unsatisfactory mechanical properties, the usage of this polymer material has been limited. In order to utilize this aliphatic polyester resin it should have a number average molecular weight of more than 30,000. However, it is difficult to manufacture the aliphatic polyester resins having a number average molecular weight of more than 15,000 using the conventional polycondensation reaction system.
In order to solve this problem, Korean laid-open patent No. 95-758 discloses the process of preparing the high molecular weight aliphatic polyester resin having a number average molecular weight of more than 30,000, by controlling the reaction temperature, the degree of vacuum and the amount of catalyst. However, this aliphatic polyester resin is difficult to mold and form due to its low weight average molecular weight and low heat-resistance.
In another method, Korean laid-open patent No. 95-114171 discloses the process of preparing the high molecular weighted aliphatic polyester resin by introducing a monomer such as a polyhydric alcohol or a poly(at least tri)-hydric carboxylic acid. According to this process, the molding and forming ability can be enhanced by introducing the monomer into the reactor for reducing the time required for the reaction and diffusing the molecules into the product. However, the utilization of the polyester resin thereof is very difficult because the physical property such as a tensile strength is poor due to the increasing amount of the low molecular weight polyesters. Furthermore, it is difficult to control the reaction for preparing the polyester resin because the polyester resin easily becomes a gel type.
In yet another process for increasing the molecular weight of the aliphatic polyester resin, Korean laid-open patent No. 95-25072 discloses the high molecular weight aliphatic polyester resin produced by an isocyanate as a coupling material reacting to an aliphatic polyester resin having a number average molecular weight of 15,000 to 20,000 which is produced by de-hydration reaction or de-glycol reaction of the mixture of main materials of (1) an aliphatic glycol (including alicyclic glycol) and (2) an aliphatic dicarboxylic acid (or anhydrous acid of it) and a little of (3) monomer of polyhydric alcohol or polyhydric carboxylic acid (or acid anhydrous thereof). According to the application, the aliphatic polyester resin has a number average molecular weight of 20,000 to 70,000. However, this process requires more time for the reaction which leads to the poor production yield. And the isocyanate used as a coupling material for increasing the molecular weight is harmful to the human body so it needs to be handled carefully.
In the conventional process as mentioned above, it has been used to introduce a coupling material like an isocyanate or a monomer such as a polyhydric alcohol or polyhydric carboxylic acid. These conventional processes have many problems such as low production yields, poor physical properties or inabilities for molding and forming.